The present invention relates to X-ray imaging devices. More particularly, the invention relates to devices for selectively imaging objects by limiting detector responses to variably selected portions of the X-ray beam.
X-rays are shortwave electromagnetic vibrations which can penetrate solid matter. They are produced when, in a vacuum, electrons are released, accelerated and then abruptly retarded. To release electrons, the tungsten filament in an X-ray tube is heated to incandescence (white heat) by passing an electric current through it. The electrons are accelerated by a high voltage (ranging from about ten thousand to some hundreds of thousands of volts) between the anode (positive) and the cathode (negative) and impinge on the anode. When the stream of very fast high-energy electrons strikes a metallic anode, the electrons are rapidly slowed down, and some of them penetrate into the metal. High energy electrons that penerate into the metal atom may dislodge one or more inner electrons of that atom. The vacant place is then taken by one of the outer electrons which thus leap from the outer to an inner "shell" and, in so doing, emit energy in the form of radiation, i.e., X-rays.
In some contemporary X-rays tubes, the anode, usually referred to as the "target", is of the rotating disk type, so that the electron beam is constantly striking a different point of the anode perimeter. The X-ray tube itself is made of glass, but enclosed in a protected casing that may be filled with oil to absorb the heat produced. The high voltage for operating the tube is supplied by a transformer with the alternating current rectified by means of rectifier tubes, or by means of barrier-layer rectifiers.
Because of their short wavelength (10.sup.-8 to 10.sup.-10 cm) X-rays can pass through objects that are opaque to ordinary light, and form shadow images of those objects on a film or fluorescent screen. Aside from the well recognized medical application of X-ray devices, they are also used to determine the mechanical integrity of structures that cannot readily be examined, such as structural members within an aircraft, or the like. In those applications X-ray devices permit the user to make an onsight inspection of, for example, a hidden structural joint, without having to remove the aircraft to a maintenance facility and disassemble outer surface members.
In order to take fullest advantage of the use of X-ray devices to perform structural examinations in field use, it is necessary that the device be readily portable and require a minimum of precise alignment before useful results can be obtained. Many contemporary X-ray devices require a fixed relation between the X-ray source and the X-ray detector, and are therefore unsuitable for many field uses. The present invention is directed to a device wherein the X-ray source and X-ray detector may be independently moved, and the device satisfactorily operated to produce X-ray imagery of an object through selective synchronization of the X-ray generating and X-ray detecting functions.
In most instances, X-rays are imaged on a film consisting of an acetate cellulose base coated with an emulsion of silver halide and gelatin. Alternatively, "live" X-ray images may be created on a fluorescent screen coated with barium platinocyanide. In yet another construction, X-ray images may be focused on a detector or array matrix formed of individual detector elements that generate an electrical voltage or current proportional to the intensity of the incident X-rays. Such a detector matrix may be scanned, or "interrogated" at a very high rate in order to produce a pattern or electrical signals representative of the X-ray pattern incident upon the matrix. That pattern may then be communicated to a monitor such as a television screen where it is illustrated for viewing.
X-ray sources typically generate X-rays in a fan-like pattern from a point source. When the point source is moved about in a pattern, each point in the pattern generates a separate fan-like pattern of X-rays. As a consequence of the movement of the X-ray point source, the X-ray beams pass through the object being X-rayed at differing angles and form shadows as ther paths overlap on route to the photographic or detection surface. The resulting images may, therefore, exhibit a lack of sharpness and uniformity. Various devices have been utilized to collimate the X-rays emitted from the point source, or otherwise enhance the sharpness of the image. Those devices include diaphragms that have narrow slits or appertures, as well as other selectively transmissive members that are disposed in front of the point source so as to restrict passage of oblique X-ray beams. Such devices typically require precise placement with respect to the point source of X-rays and the detection surface. Consequently, those devices are inadequate for many field uses in that they lack the flexibility to vary the angle at which the incident X-rays may be directed and observed in order to get a clearer picture of an irregular shaped object.
The present invention is directed to addressing those and other deficiencies in providing an X-ray imaging system that is portable and is adapted to readily permit selective imaging of X-rays impinging the detector surface from different angles so as to enhance the image quality, and reduce shadows generated by the interaction of overlapping X-ray beams.